Method and device for processing waste paper

ABSTRACT

The invention relates to a method for processing waste paper according to the following steps: a) the waste paper is decomposed into fibres, b) the fibres are washed, c) the fibres are subjected to a dispersion process, d) the fibres are subjected to a flotation method, e) the fibres are processed for further treatment in a paper machine. The liquid from the fibres obtained in step b) is reintroduced into the fibre flow prior to step d). The invention also relates to a device for carrying out said method.

The present invention relates to a method and a device for processing waste paper.

Nowadays, paper which meets high quality requirements can be produced from waste paper. A particular problem here is the removal of printing inks from the waste paper (“deinking”). Flotation processes are used for this purpose in which hydrophobic ink particles in a specific particle size range are separated from the isolated paper fibres and discarded. In this case, all the fibres should be retained as far as possible. Usually all those printing inks used in the most important printing methods, i.e. intaglio and offset printing, can be removed by this flotation process.

In addition, other printing processes are gaining in importance which use printing inks which cannot be removed by the commonly used flotation process because they are not released from the fibre or because they form too-large or too-small or hydrophilic ink particles. For example, UV-cross-linked inks and vegetable-oil-based inks form too-large particles. For example, toner or flexographic printing inks and ink jet printing inks form too-small particles. In order to solve the problem of the non-released or too-large ink particles, another process stage is required in which the fibres tainted with these printing inks are dispersed. In this case, the ink particles are released from the fibres and at the same time comminuted. As a result, they can be separated from the fibres in a subsequent step by a usual flotation process. This additional process stage is associated with an appreciable loss of fibres.

EP 1 731 662 A1 describes a process for processing waste paper in which the isolated fibres are fractionated after the first flotation step, i.e. divided into a short fibre fraction and a long fibre fraction. The short fibre fraction substantially contains short fibres and other fine substances and fillers. In addition to the long fibres, the long fibre fraction contains substantially larger impurities such as adhesive particles (“stickys”), agglomerates and non-released printing inks still adhering to the long fibres. Only the long fibre fraction undergoes dispersion. The two fractions are combined again in the further course of the process. In this case, the fibre losses are reduced and energy is saved because the short fibre fraction is not co-dispersed. In addition, the machine size for the dispersion can be reduced.

A further development of this process is also known in which the fractionation takes place before the first flotation process. After the long fibre fraction has been dispersed and possibly purified by a sorting process, both fractions are each subjected to a single flotation process separately. The hitherto necessary second flotation process is omitted completely so that energy is additionally saved and the two installations for the flotation process can each be made smaller than hitherto.

However, this further development has the disadvantage that during the fractionation the ink particles already released from the fibres are still completely present since the hitherto usual first flotation process is omitted. Consequently, the long fibre fraction can contain ink particles which have already been released and which are entrained in the course of the processing method. These ink particles are then further comminuted during the dispersion so that as a result of their then too-small particle size, they can no longer be separated from the fibrous material or the fibre suspension by means of the subsequent flotation process. The dispersion has the disadvantageous effect on the particles which have already been released that they are absorbed on the fibres again. Another problem is that as a result of the separation into a long-fibre fraction and a short fibre fraction, the quality of the resulting paper can be unsatisfactory.

It is therefore the object of the present invention to further develop a generic method or a generic device in such a manner that all the ink particles are reliably removed and the quality of the resulting paper is as far as possibly constantly high.

The solution consists in a method having the features of patent claim 1 and a device having the features of patent claim 12. According to the invention, it is provided that the waste paper is decomposed into fibres, the fibres are subjected to a washing process, a dispersion process, and a flotation process, and are then processed for further treatment in a paper machine, wherein the liquid accumulating in step b) is supplied to the fibres again before step d).

The term washing or washing process is defined as a solid-solid separating process in which fine particles are separated from the fibres. Suitable washing stages for this purpose are known as such. A suitable known configuration of such a washing stage comprises, for example, a central roller with a revolving screen. The fibre stream is guided in a material distributor and sprayed through a nozzle into the intermediate space between the rotating central roller and the revolving screen. The fibre stream is thickened by the screen tension and the resulting filtrate containing the fine particles is hurled away as a result of the centrifugal force and guided outwards. The washed and thickened fibres are removed from the central roller and further processed.

Therefore, the released ink particles already present in the fibre stream and any fillers which may be present are reliably separated from the fibres. The fibres obtained in the course of the process are particularly well suited for producing high-quality papers because the degree of whiteness and cleanliness are appreciably improved. In addition to the printing inks already released, the filtrate obtained during the washing also contains fine substances and short fibres which are important for the paper quality. It is therefore provided according to the invention to return this filtrate into the fibre stream before the flotation stage. In this way, both the long fibres and the short fibres are guided through the dispersion stage in a single fibre stream, where the remaining printing inks are released from the fibres. The printing ink particles which have already accumulated in the filtrate of the washing stage after the washing are guided together with the important fine substances and short fibres around the dispersion stage and fed to the flotation stage. In this way it is ensured that all the printing inks are removed during the flotation whilst at the same time the loss of short fibres and fine substances is minimised. In this way a substantially improved paper quality is achieved and at the same time, the machine expenditure for processing the waste paper is significantly reduced.

A further advantage of the method according to the invention is that only one flotation stage is required which is situated towards the rear, i.e. downstream in the process sequence compared with the prior art. The material losses resulting from the second flotation stage hitherto necessary in the prior art no longer occur in the method according to the invention.

A further important aspect of the present invention consists in that a fractionation of the fibres into a short fibre fraction and a long fibre fraction hitherto deemed to be essential in the prior art is omitted. For the person skilled in the art it was surprisingly established that passing the combined short and long fibres through a single washing stage, a single dispersion stage and a single flotation device whilst simultaneously guiding the filtrate obtained in the washing stage around the dispersion stage is only associated with a low fibre loss. Almost the entire amount of fibres obtained when decomposing the waste paper and a significant fraction of the fine substances essential for the paper quality ultimately reach the paper machine to produce new paper. As a result, an appreciable improvement in the quality of the resulting paper is achieved.

In addition, the method according to the invention is substantially simplified compared with the prior art so that the device according to the invention suitable for carrying out the method has a substantially less complex structure than the apparatus known in the prior art. This is therefore naturally accompanied by considerable cost savings.

Advantageous further developments are obtained from the dependent claims.

After the washing and before the dispersion, a further thickening, for example, a high-consistency thickening, can take place to further reduce the amount of the fibre stream guided through the dispersion stage. This is particularly appropriate if an optimal material density is to be set for the dispersion so that the best possible shear effect can be achieved.

The liquid accumulating during the thickening can either be removed or, in the same way as the filtrate obtained in the washing stage, fed back into the fibre stream before the flotation stage.

The fibres obtained during the decomposition of the waste paper can be subjected to at least one usual sorting process before the washing, for example, a coarse sorting or a fine sorting in order to remove impurities which may still be present.

A feasible processing of the fibres for further treatment in a paper machine can, for example, consist in at least one thickening step in order to reduce the volume of the fibre stream.

A further advantageous embodiment of the method according to the invention consists in subjecting the filtrate accumulating in the washing stage to a separate thickening and using the water accumulating in this separate thickening to optimally adjust the consistency of the fibre stream upstream of the washing stage in order to further improve the fibre quality. The material accumulating during the separate thickening is then fed back into the fibre stream again before the flotation stage.

In a further development of this embodiment, the fibrous material obtained after the dispersion can be used as filter aid for the separate thickening. In this case, it is advantageous to dilute the fibre stream again before flotation since the material accumulating during the separate thickening in combination with the fibrous material used as filter aid is possibly too viscous. The water possibly accumulating during the additional thickening stages provided before the actual paper manufacture is, for example, suitable for this purpose. The combination of these advantageous measures leads to the configuration of two water circuits by which means not only the fibre quality can be optimised but also the water loading of the method according to the invention and the volume of the accumulating waste water can be reduced.

Exemplary embodiments of the present invention are described in detail hereinafter with reference to the appended drawings. In the figures:

FIG. 1 shows a block diagram of the first embodiment of a method according to the invention;

FIG. 2 shows a block diagram of a second embodiment of a method according to the invention.

The block diagrams show two embodiments of the method according to the invention in its entirety. The machines used for this purpose and other devices as such are known to the person skilled in the art so that their detailed description is omitted.

FIG. 1 shows a first embodiment of the method according to the invention. The waste paper is defibered, i.e. decomposed into fibres in a decomposition stage 10 in a manner known per se. The fibres obtained are present in a fibrous material mixture containing short fibres and long fibres, wherein the short fibres are already freed from printing inks whereas printing inks which have not yet been released can adhere to the long fibres. The fibrous material mixture therefore contains released ink particles and in addition other fine materials and fillers as well as coarser impurities such as adhesive particles (“stickys”) and non-paper components. In the exemplary embodiment the fibrous material mixture is therefore subjected to a coarse sorting in a first sorting stage 11 in a manner known per se and if necessary, a fine sorting in a second sorting stage 16 in order to separate impurities such as adhesive particles or non-paper components from the fibres. According to the invention, the complete fibre stream containing short fibres and long fibres is then supplied to a washing stage 17 where it is subjected to washing. During the washing fine solid particles such as fillers and printing inks which have already been released but possibly also fine substances and some short fibres are separated from the fibre stream. The long fibres and some of the short fibres are retained in the fibre stream.

After the washing, it is appropriate to subject the fibre stream, for example, to a high-consistency thickening in a thickening stage 19 in order to bring it to a suitable consistency for the dispersion. The fibre stream is then dispersed in a dispersion stage 21 in a manner known per se. At the same time, the printing inks still adhering to the fibres, in particular the long fibres, are released in the form of small ink particles. The ink particles are now accessible to a flotation process whereby they can be separated from the fibres. The fibres can, for example, be oxidatively bleached at the same time as the dispersion.

Following the dispersion and optionally the bleaching, the fibre stream is subjected to a flotation process in a flotation device 23 in a manner known per se. In this case, it is provided according to the invention that the filtrate accumulating during the washing in the washing stage 17 is removed from the washing stage 17 via a line 27 and is fed back into the fibre stream before the flotation stage 23. The printing inks which have already been released and are contained in the filtrate are in this way passed around the dispersion stage 21 with the valuable fine substances. Dispersion of the filtrate is not required and even undesirable since the printing inks contained in the filtrate are already present in dispersed form and no longer adhere to the fibres. In this way, the qualitative result of the dispersion of the remaining fibre stream in the dispersion stage 21 is improved. At the same time, the dispersion stage 21 can be designed for a lower volume of fibre stream than was hitherto necessary in the prior art.

It is possible to proceed in a comparable manner with the water accumulating during the thickening in the thickening stage 19 in that in the exemplary embodiment, this is led off via the line 28 and fed back to the fibre stream together with the filtrate accumulating in the washing stage 17 before the flotation stage 23.

During the flotation in the flotation stage 23, all the hydrophobic solids having particles sizes lying approximately between 5 μm and 500 μm in diameter are separated from the fibres in a manner known per se. This includes in particular all the printing inks which are present in dispersed form in the fibre stream after the dispersion stage 21 or are fed back to the fibre stream upstream of the flotation stage via the lines 27, 28.

The fibre stream thus purified can then be thickened in a thickening device 24, for example, a disk filter and optionally thereafter the fibres are bleached. In the exemplary embodiment this is then followed by a high-consistency thickening in a further thickening stage 25. The resulting fibre fleece can then be fed to a paper machine. This paper machine can in particular be a sheet forming unit which can produce single- or multi-ply paper products, for example, two-ply corrugated board or four-ply cardboard.

In the embodiment shown as an example in FIG. 1, the method according to the invention achieves the best results, i.e. processed waste paper fibres of good to very good quality, particularly when the consistency of the fibre stream before the washing stage 17 and before the flotation stage 23 is approximately the same. This means that when returning the filtrate from the washing stage 17 or the liquid from the thickening stage 19, the consistency of the fibre stream can be optimally adjusted before the flotation stage 23.

For reasons of quality, the fibre stream can be present in a particularly thin consistency before the washing stage 17 with the result that the resulting filtrate is thinner than desired and on account of quality considerations cannot be completely returned to the fibre stream before the flotation stage 23. If such a water excess exists, it can be appropriate to modify the method according to the invention, as shown as an example in FIG. 1, in such a manner that an embodiment of the method according to the invention as shown as an example in FIG. 2 results.

FIG. 2 shows a second embodiment of the method according to the invention which corresponds to the embodiment according to FIG. 1 in its essential steps. The process stages identical to the exemplary embodiment shown in FIG. 1 are therefore provided with the same reference numerals. The waste paper is likewise guided via a decomposition stage 10, a first sorting stage 11 and optionally a second sorting stage 16. The resulting fibre stream is further processed in the same way as was described with reference to FIG. 1. The fibre stream is therefore supplied to a washing stage 17 where it undergoes washing. The fibre stream is then subjected to a high-consistency thickening process in a thickening stage 19 and then dispersed in a manner known per se in a dispersion stage 21. At the same time as the dispersion, the fibres can, for example, be oxidatively bleached. The fibre stream is then subjected to a flotation process in a flotation device 23 in a manner known per se. The purified fibre stream is thickened in a thickening device 24, for example, a disk filter. This is followed by a high-consistency thickening in another thickening stage 25. The resulting fibre fleece can now be supplied to a paper machine.

The modification of the method according to the invention now consists in that the consistency of the fibre stream, i.e. the degree of dilution, can be optimised in each phase of the method according to the invention. The starting point for the modification is the consideration that for reasons of quality, the fibre stream can be present in a highly diluted form before the washing stage 17. The filtrate resulting from the washing stage 17 therefore accumulates in a comparatively large quantity. Nevertheless, the important short fibres and fine substances contained in this filtrate, as described above, should not be lost. Consequently, the filtrate extracted from the washing stage 17 by means of the line 18 is subjected to a separate thickening in a separate thickening stage 31. By analogy to the embodiment of the method according to the invention described in FIG. 1, the thickened filtrate is returned to the fibre stream before the flotation stage 23. The thickened filtrate contains the important short fibres and fine substances and similarly to the method according to the invention described in FIG. 1, the released printing inks which have already accumulated during the defibering and sorting, which are finally removed in the flotation stage 23. The water accumulating in the thickening stage 31 can now be used to optimally adjust the consistency of the fibre stream before the sorting stages 10 and/or 11 and/or before the washing stage 17 in order to ultimately obtain a particularly good fibre quality.

The thickening devices known in the prior art operate with a filter aid. A further modification of the method according to the invention provides that the fibre stream resulting from the dispersion stage 21 which has a suitable consistency is used for this purpose. To this end the fibre stream is partially or completely withdrawn after the dispersion stage and fed via a line 32 to the thickening stage 31 where the short and long fibres contained in the fibre stream are used as filter aid, for example, as supporting fibres in a vacuum disk filter known per se. The person skilled in the art is naturally free to use a separate filter aid instead of this.

If the fibre stream is used as filter aid, all the fibres obtained in the thickening stage 31 such as fine substances, fillers and dispersed printing inks, are fed back to the fibre stream and subjected to a flotation in the flotation stage 23. For this purpose the fibre stream must optionally be diluted again before the flotation stage 23. In this case, a further modification of the method according to the invention provides that the water obtained in the thickening stages 24 and/or 25 is used for this purpose and is returned into the fibre stream via the line 33 before the flotation stage 23.

The combination of all these modifications results in an embodiment of the method according to the invention which comprises two water circuits. A second, smaller, water circuit leads from the last thickening stages 24 and/or 25 before the flotation stage 23 and back. The water contained therein is always cleaner than the water contained in the first, larger, water circuit. This first water circuit leads from the washing stage 17 or thickening stage 19 via the thickening stage 31 upstream of positions located before the washing stage 17, for example, before the sorting stages 10 and/or 11 and/or before the washing stage 17. In this way, the water loading of the method according to the invention is reduced. In each stage of the method according to the invention, the consistency of the fibre stream can be optimally adjusted. If it is necessary to replace contaminated water by clean water in the circuits, the comparatively clean water present in the second smaller circuit can be discharged into the first larger circuit whose contaminated water is fed to a clarification plant.

The water extracted from the second smaller circuit in this way can be replaced by clean fresh water (not shown). Consequently, not only the water loading of the method according to the invention is reduced but the volume of waste water accumulating in a clarification plant for purification is also reduced. 

1. A method for processing waste paper comprising the following steps: a) the waste paper is decomposed into fibres, b) the fibres are subjected to a washing process, c) the fibres are subjected to a dispersion, d) the fibres are subjected to a flotation process, e) the fibres are processed for further treatment in a paper machine, wherein the liquid accumulating in step b) is supplied to the fibre stream again before step d).
 2. The method according to claim 1, characterised in that after step b) and before step c), the fibres are subjected to at least one thickening.
 3. The method according to claim 2, characterised in that the liquid accumulating during the at least one thickening is removed.
 4. The method according to claim 2, characterised in that the liquid accumulating during the thickening is supplied to the fibres again after step c).
 5. The method according to claim 1, characterised in that the fibres are subjected to at least one sorting before step b).
 6. The method according to claim 5, characterised in that the fibres are subjected to a coarse sorting and a fine sorting before step b).
 7. The method according to claim 1, characterised in that the fibres are subjected to at least one thickening after step d).
 8. The method according to claim 7, characterised in that the water accumulating during the at least one thickening carried out after step d) is supplied back to the fibre stream before step d).
 9. The method according to claim 1, characterised in that at least the liquid accumulating in step b) is subjected to a separate thickening and the thickened material is supplied to the fibre stream again before step d).
 10. The method according to claim 9, characterised in that the fibre stream accumulating after step c) is partially or completely used as filter aid for the separate thickening.
 11. The method according to claim 9, characterised in that the water accumulating during the separate thickening is supplied to the fibre stream again upstream of step b).
 12. A device for processing waste paper having the following features: a) a decomposition stage for decomposing the waste paper into fibres, b) a washing stage for washing the fibres, c) a dispersion stage for dispersing the fibres, d) a flotation device for flotation of the fibres, e) at least one processing stage for processing the fibres for further treatment in a paper machine, wherein a line is provided for the liquid accumulating in the washing stage, which branches off from the washing stage and opens into the fibre stream again before the flotation device.
 13. The device according to claim 8, characterised in that at least one thickening stage is provided between the washing stage and the dispersion stage.
 14. The device according to claim 9, characterised in that a line for the liquid accumulating in the thickening stage branches off from the at least one thickening stage and opens into the fibre stream again before the flotation device.
 15. The device according to claim 8, characterised in that at least one sorting stage is provided before the washing stage.
 16. The device according to claim 11, characterised in that a coarse sorting stage and a fine sorting stage are provided.
 17. The device according to claim 8, characterised in that at least one thickening stage is provided after the flotation device.
 18. The device according to claim 17, characterised in that a line for the extracted water branches off from the at least one thickening stage and opens into the fibre stream before the flotation stage.
 19. The device according to claim 8, characterised in that a separate thickening stage is provided at least for the liquid accumulating in the washing stage.
 20. The device according to claim 19, characterised in that a line for at least some of the fibre stream accumulating after the dispersion stage is provided after the dispersion stage and opens into the thickening stage.
 21. The device according to claim 19, characterised in that a line branches off from the thickening stage and opens into the fibre stream at least one point upstream of the washing stage. 